The present invention relates to a dynamic pressure-type liquid bearing unit used in a spindle motor or the like and a magnetic disk device mounted with the same, and in particular, it relates to a means for preventing a lubricant from leaking.
FIG. 5 shows a prior-art liquid bearing unit.
One end of a stationary shaft 2 with dynamic pressure-generating grooves 4a and 4b formed on the outer circumferential surface thereof is press-fitted into a base 1, and on the other end thereof, a thrust plate 3 is fitted, thus a shaft portion is formed. A sleeve 5 is press-fitted to the inner circumferential surface of a hub 6 for attachment of a magnetic disk or the like, and a thrust flange 11 is attached to one end of this sleeve 5, thus a rotator is formed. Moreover, the stationary shaft 2 is inserted into a bearing hole of the sleeve 5 so that the thrust flange 11 and thrust plate 3 are opposed to each other, and a lubricant 10 is filled between the shaft portion and rotator, thus the rotator is rotatably supported by the shaft portion.
A stator coil 9 is provided on a wall formed on a base 1, a rotor magnet 7 is attached via a rotor yoke 8 to a surface of the hub 6 opposed to the stator coil 9, thus a motor driving portion is constructed.
When the sleeve 5 and the hub 6 are driven to rotate by this motor driving portion, a dynamic pressure is generated in the lubricant 10 by pumping action of the dynamic pressure-generating grooves 4a and 4b formed on the stationary shaft 2, and the shaft portion and the rotator are supported to rotate without contact.
The lubricant 10 filled between the shaft portion and rotator is retained inside the bearing by surface tension, however, if the lubricant 10 scatters or oozes out due to a rotating drive of the bearing unit, a rotation failure of the bearing or contamination to the exterior occurs. In order to prevent such a leak of the lubricant 10, an oil repellent film 12 having the property to repel the lubricant 10 is formed on the lower end surface of the sleeve 5.
This oil repellent film 12 has been formed from a point of view where a film that moderately repels the lubricant 10 suffices, and the ability to repel the lubricant 10 (hereinafter, referred to as xe2x80x9crepelling abilityxe2x80x9d) has not been particularly regarded as important. For example, in a liquid bearing unit used in an optical disk device, fluorine-based lubricant 10 which is excellent in heat resistance and chemical stability and whose change in kinematic viscosity according to temperature is small has been used, and as the oil repellent film 12, a silicone-based oil repellent film 12 which easily repels this fluorine-based lubricant 10 has been used.
However, with the realization of high-speed rotation of bearing units in recent years, in a liquid bearing unit provided with the above prior-art oil repellent film 12, surface tension of the lubricant 10 has declined due to a rise in temperature as a result of heat generation from the motor portion and frictional heat in the bearing portion and an increase in centrifugal force to be applied to the lubricant 10 has become remarkable. Therefore, in the prior-art liquid bearing unit, it has been impossible to sufficiently prevent a leak of the lubricant 10 from the bearing, this has induced scattering or oozing of the lubricant 10, thus easily causing a rotation failure of the bearing and contamination to the exterior.
In addition, such a liquid bearing unit using the silicone-based oil repellent film 12 as in the above has a problem such that this can be mounted on an optical disk device, but cannot be applied to a magnetic disk device. This is because if the liquid bearing unit using the silicone-based oil repellent film 12 is mounted on a magnetic disk device, polysiloxane and the like which is generated from the silicone film adsorbs to a magnetic disk or a head and exerts negative influences on the sliding movement thereof, which may cause a reading/writing error or a head crash.
It is an object of the present invention to solve the above problems and provide a highly-reliable liquid bearing unit which can prevent a lubricant from leaking by securely repelling the lubricant by an oil repellent film, and a magnetic disk mounted with the same.
A liquid bearing unit of the present invention is characterized by a construction such that combinations of a lubricant and an oil repellent film are controlled and surface tension of the lubricant becomes greater than the critical surface tension of the oil repellent film by an appropriate value.
According to the present invention, even when a rise in temperature or an increase in centrifugal force occurs in the bearing portion due to high-speed rotation, a leak of the lubricant can be reduced by maintaining the repellent ability of the oil repellent film.
A magnetic disk device of the present invention is characterized by a special construction for a lubricant and an oil repellent film.
According to the present invention, a highly reliable magnetic disk device without a reading/writing error or a head crash can be realized.
A first mode according to the present invention is a liquid bearing unit in which a lubricant is filled between a shaft portion and a rotator which is rotatably supported by this shaft portion and an oil repellent film for preventing the lubricant from leaking is formed on the shaft portion or rotator in the vicinity of a gas-liquid interface of the lubricant, wherein the lubricant contains a constituent of at least one of a hydrocarbon-based oil and an ester-based oil, the oil repellent film is a film of at least one of a fluorine base and a silicone base, and surface tension of the lubricant is made greater than the critical surface tension of the oil repellent film by 5 mN/m or more. Thus, by providing a fixed difference or mode between the critical surface tension of the oil repellent film and surface tension of the lubricant, despite a decrease in the surface tension of the lubricant due to a rise in temperature as a result of heat generation from the motor portion and frictional heat in the bearing portion and an increase in centrifugal force due to high-speed rotation, scattering or oozing of the lubricant can be securely prevented, whereby a highly reliable liquid bearing unit can be realized.
A second mode according to the present invention is a fluid bearing unit in which a lubricant is filled between a shaft portion and a rotator which is rotatably supported by this shaft portion and an oil repellent film for preventing the lubricant from leaking is formed on the shaft portion or rotator in the vicinity of a gas-liquid interface of the lubricant, wherein the lubricant is a fluorine-based lubricant, and surface tension of the lubricant is made greater than the critical surface tension of the oil repellent film by 8 mN/m or more. Thus, by providing a fixed difference or mode between the critical surface tension of the oil repellent film and the surface tension of the lubricant, despite a decrease in the surface tension of the lubricant due to a rise in temperature as a result of heat generation from the motor portion and frictional heat in the bearing portion and an increase in centrifugal force due to high-speed rotation, scattering or oozing of the lubricant can be securely prevented, whereby a highly reliable fluid bearing unit can be realized.
In addition, in the fluid bearing unit according to the second mode, when the fluorine-based lubricant contains at least one of perfluoropolyether and perfluoropolyether derivatives, a fluid bearing unit which withstands use in a high temperature range and whose torque change according to temperature is small can be realized. In addition, when the oil repellent film is a coating film, an oil repellent film can be easily formed at low cost, and moreover, by dispersing and dissolving various materials in the coating solution, other functions can be easily added after formation of the oil repellent film. Furthermore, when the oil repellent film is a fluorine-based film, a fluid bearing unit which can also be mounted on a magnetic disk device can be realized.
A third mode according to the present invention is a magnetic disk device mounted with a fluid bearing unit in which a lubricant is filled between a shaft portion and a rotator which is rotatably supported by this shaft portion and an oil repellent film for preventing the lubricant from leaking is formed on the shaft portion or rotator in the vicinity of a gas-liquid interface of the lubricant, wherein the lubricant is a fluorine-based lubricant, the oil repellent film is a fluorine-based film, and surface tension of the lubricant is made greater than the critical surface tension of the oil repellent film by 8 mN/m or more. Thus, a highly reliable magnetic disk device without a reading/writing error or a head crash can be realized.